GPS signal locks up

I have a tomtom one (30 series) which finds a good gps signal for 5/6 satellites then locks up after 5 mins or so. The little blue/yellow circle top right of gps status screen stops turning. All the satellites remain blue and the map stays in colour but no cursor movement. Have tried different nc's and maps, ephemeris downloads all to no avail. Am thinking the gps module must be defective but it always searches and finds satellites before freezing. Has anyone encountered anything similar or have any suggestions please?

DeviceName=TomTom ONE
DeviceVersionHW=ONE (v8)
RamDiskVersion=20080529
BootLoaderVersion=55281
LinuxVersion=190943
ApplicationVersionVersionNumber=8010
ApplicationVersion=9369

CurrentMap=United_Kingdom_and_Republic_of_Ireland_890_4234
CurrentMapVersion=890.4234

Has anyone changed a GPS module on one of these and can tell me how easy/difficult?